Add the ability to export the graph as an image

[darrylnoakes]

Please add the ability to export the graph as an image.

For example, regexper.com supports this, providing download buttons for SVG and PNG.

SVG alone would be good enough, but PNG as well would be even better, if you care to.

[Bowen7]

Thanks for your feature request. I will add this feature either after or during the refactor. You can find more information about the refactor at #105.